1. Field of the Invention
The present invention relates to nitrogen removal, and more particularly, to an aquaculture system including a denitrification unit for denitrifying aquatic effluent by anaerobic bacteria in combination with a nitrification unit.
2. Description of the Related Art
The development of technology and protocols that will enable the aquaculture industry to expand production of aquatic species in urban recirculating systems requires systematic examination of each aspect of said aquatic systems. Many conventional aquaculture systems are known and can be classified in one of two ways; closed and open systems. Natural environments, such as lakes and streams, most easily typify open systems. Closed systems may be defined as completely closed with constant recirculation and a quasi-closed system can include some recirculation and a portion of the output may be recycled as part of the new input. Regardless of which system is utilized, optimal growth of the aquatic species is directly related to the environmental parameters, and as such, pollutants and waste by-products must be removed from the system to assure the species viability.
In most closed and quasi-closed aquaculture systems, ammonia is oxidized to nitrite NO2− in an aerobic biofilter by autotrophic bacteria. Nitrate is more toxic then the ammonium ion, so a second bacterium is required to oxidize the nitrite to nitrate. While nitrate is considerably less toxic than ammonium or nitrate, it can also be a problem. Nitrate is typically removed from recirculating culture systems by water exchange. However, water exchange has several drawbacks. First, water removal in aquaculture systems normally involves a slow exchange with a thorough mixing of old and new water to avoid stressing the cultured aquatic species. Second, in systems where natural sea water is unavailable, deionized water and sea salts must be mixed, which may incur heavy additional costs. Finally, the high nitrate effluent must be normally discharged.
However, the discharge of the high nitrate effluent is potentially a problem, since salt water cannot typically be discharged into a sewage system or a fluvial system. Further, there is a growing environmental concern about the discharge of nitrogenous waste. Discharge permits may be complex and often require very stringent pollutant limits. Thus, attempts have been made to denitrify the discharge. However, because denitrification is exclusively an anaerobic process, the process is rarely included in any biofiltration systems that require oxygenated water, as is the case in aquaculture systems. In order to achieve anaerobic conditions that would stimulate denitrification, many filtration systems require the addition of organic compounds to promote oxygen consumption during degradation by heterotrophic bacteria (and induce anaerobic pockets), as well as to serve as electron donors to support biological nitrate reduction in denitrifying biofilters. Additionally, alcohols, volatile fatty acids and sugars, which are often used as carbon sources for such systems, often lead to bacterial blooms, toxic by-products, among other problems, and result in elevated system costs. Thus, the disadvantages of stimulating denitrifying activity often outweigh the advantages. Accordingly, it would be advantageous to develop a system and method that uses denitrification as a viable means of nitrate removal and that addresses at least some of the shortcomings of the prior art systems.